Direct viewing storage tube having mesh halftone target and nonmesh bistable target

ABSTRACT

A direct viewing charge image storage tube is described in which a low density halftone storage target is employed along with a higher density bistable storage target. Charge images are written on the halftone target at a fast writing speed, up to about 300 million centimeters per second, and then such charge image is transferred to the bistable target where it is stored for a long time, up to one hour or more. The bistable storage target may be of the nonmesh type that employs a storage dielectric of phosphor material which emits a light image corresponding to the charge image stored thereon. The storage dielectric of a halftone target is more porous and lower density than the phosphor storage dielectric and may be a porous metal oxide, such as magnesium oxide, having a density of 2 to 5 percent of its normal bulk density which provides a target of low capacitance and extremely fast writing speed.

United States Patent 1 11 3,710,173

Hutchins, IV et al. 1 Jan. 9, 1973 [s41 DIRECT VIEWING STORAGE TUBE 3,531,675 9/1910 Frankland .smss R HAVING MESH HALFTONE TARGET AND NQNMESH BISTABLE TARGET Primary Examiner-Benjamin A. Borchelt Assistant Examiner-S. C. Buczinski [75] Inventors: Thomas B. Hutchina, IV, Portland, Almmey auckhomy more marquis; sparkman 0reg.; William M. Templeton, Seattle, Wash. 57 ABSTRACT Assign: TekmmhrlncqBeavenonvomg- A direct viewing charge image storage tube is [22] Fnad: June 17, 1970 described in which a low density halftone storage target is employed along with a higher density bistable PP 47,005 storage target. Charge images are written on the halftone target at a fast writing speed, up to about 300 [52] CL 315/12 315/13 ST 3/68 R million centimeters per second, and then such charge 3/68 A 313/92 313/68 image is transferred to the bistable target where it is [5]] Int. Cl. .j lolj 29/41 stored for a long time up to one hour or more The bistable storage target may be of the nonmesh type [58] Field of Search 13 68 68 that employs a storage dielectric of phosphor material 313/68 92 R which emits a light image corresponding to the charge image stored thereon. The storage dielectric of a half- [56] References Cited tone target is more porous and lower density than the UNITED STATES PATENTS phosphor storage dielectric and may be a porous metal oxide, such as magnesium oxide, having a densi- 3,l65,664 1/1965 Callick...................................3l5/l2 ty of 2 to 5 percent of its normal bulk density which 3.293.473 12/1966 Anderson provides a target of low capacitance and extremely 3,197,661 Sinclair fast writing speed 3.213316 lO/l965 Goetze etal...... 3,293,474 l2/1966 Gibson, Jr ..3l3/92 R 14 Claims, 6 Drawing Figures 24 I4 50V n Y K 'IKV M HALF-TONE Bl STABLE STORAGE STORAGE PATENTEDJAN 9191a WILLIAM M. TEMPLETON THOMAS B, HUTCHINS DZ INVENTORS.

BUCKHORN, BLORE, KLARQUIST a. SPARKMAN ATTORNEYS HOR.

RAMP

GEN

HALFTONE TARGET TRIGGER GEN DIRECT VIEWING STORAGE TUBE HAVING MESH I-IALFTONE TARGET AND NONMESI'I BISTABLE TARGET BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to charge image storage tubes, and in particular to charge transfer storage tubes employing two storage targets including a halftone storage target of low density and a bistable storage target of higher density. The bistable target may be of a nonmesh type having a storage dielectric of phosphor material which emits a light image corresponding to the charge image stored thereon. The storage tube of the present invention also includes transfer means for transferring the charge image written on the halftone storage target to the bistable storage target for longer storage by using low-velocity flood electrons.

The halftone storage target in the present tube is made of a more porous and lower density dielectric than the bistable target so that such halftone target is capable of an extremely high writing speed. Thus, the charge image of the input signal may first be written on the halftone target by deflecting the writing beam with such input signal and then such charge image is subsequently transferred to the bistable target by the flood electrons for storage. As a result, the tube of the present invention has the advantages of both the high writing speed of a conventional halftone storage tube and the long retention time of a conventional bistable storage tube without the disadvantages of such tubes. Furthermore, since the halftone target can be optimized for fast writing speed while the bistable storage target can be optimized for long storage time, the tube of the present invention actually outperforms such conventional tubes.

It has previously been proposed to make a direct viewing storage tube having a low capacitance, fast writing mesh target, and a high capacitance, long storage time mesh target positioned in front of a separate phosphor viewing screen, as shown in U.S. Pat. No. 3,165,664 of E. B. B. Callick, issued Jan. 12, I965. The tube of the present invention is simpler and less expensive than such prior art tube since it does not employ a mesh-type storage target for the bistable target or a separate phosphor viewing screen. In addition, since the meshes of the two storage targets of the prior art are close together and of substantially the same size, at some potentials moire" patterns are produced in the electron image which tend to distort the display produced on the fluorescent screen, and are avoided in the tube of the present invention. In such prior art tube, the secondary electrons emitted from the bistable target are collected by a collector electrode on the other side of the halftone target so that the field of the halftone target tends to prevent efficient collection and a fourth mesh electrode may be required between the halftone target and the bistable target. No such additional collector mesh electrode is required in the present tube because the secondary electrons emitted from the bistable target are collected by its own target electrode.

It has previously been proposed in U.S. Pat. No. 3,293,473 of R. H. Anderson, issued Dec. 20, 1966, to provide a direct viewing bistable storage tube with a phosphor storage dielectric coated over a light-transparent, conductive target electrode. While this storage tube is of simple construction and provides good bistable storage, it does not have as high a writing speed as a conventional halftone storage tube or the tube of the present invention. In addition, it has also been proposed in U.S. Pat. No. 3,213,3l6 of G. W. Goetze et al., issued Oct. 19, 1965, to provide a halftone storage tube of a television camera with a storage dielectric of porous insulating material, such as magnesium oxide, to amplify the voltage of the charge image due to electron multiplication within the target. However, this storage tube is not a direct viewing storage tube and only produces a readout in the form of an electrical readout signal by scanning the storage dielectric with a reading beam of electrons.

It is therefore one object of the present invention to provide an improved direct viewing storage tube of simple and inexpensive construction having two storage targets with a more porous storage dielectric on one than the other of the targets so that such one target has an extremely fast writing speed while the other target has a long storage time.

Another object of the invention is to provide such a storage tube with a bistable storage target of nonmesh type having a phosphor storage dielectric which is employed, along with a halftone storage target of the mesh type and means for writing a charge image on the halftone target, for transferring the charge image to the bistable storage target and for storage and display of such charge image on the bistable target.

A further object of the present invention is to provide such a storage tube which is capable of halftone storage and bistable storage, as well as such transfer storage operation.

Still another object of the invention is to provide such a storage tube in which the storage dielectric of the halftone target is made of a porous insulating material having a density less than l0 percent of its normal bulk density to provide an extremely low capacitance and high writing speed.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a diagrammatic view showing one embodiment of the storage tube of the present invention and associated electrical circuitry;

FIG. 2 is a horizontal section view taken along the line 2-2 of FIG. 1 showing a portion of the tube on an enlarged scale;

FIG. 3 is an elevation view of another embodiment of the bistable storage target in the tube of FIG. 1;

FIG. 3A is a vertical section view taken along the line 3A--3A of FIG. 3;

FIG. 4 is an elevation view of a third embodiment of the bistable storage target employed in the storage tube of FIG. I; and

FIG. 4A is a vertical section view taken along the line 4A-4A of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, one embodiment of the direct view storage tube of the present invention includes a first halftone storage target 10 and a second bistable storage target 12 which are mounted within one end of an evacuated envelope 14. An electron gun l6, hereafter referred to as the writing" gun, is also mounted in the envelope 14 at the opposite end thereof and includes a cathode 18 connected to a highly negative D.C. voltage source of about I kilovolt. A narrow focused beam of high-velocity electrons is emitted by the writing gun l6 and transmitted between a pair of horizontal deflection plates 20 and a pair of vertical deflection plates 22 onto the storage targets 10 and I2 to produce charge images thereon. In addition, a pair of flood" electron guns 24 having grounded cathodes 26 are provided within the envelope 14 in a position so that they uniformly bombard the storage targets 10 and 12 with low-velocity flood electrons.

The halftone storage target 10 is a mesh-type target including a mesh target electrode 28 of metal coated with a storage dielectric 30 of highly porous insulating material to provide a low-capacitance target which is capable of an extremely high writing speed but only retains the charge image formed thereon a very short time of a few seconds. The mesh target electrode 28 may be formed of a woven wire mesh of stainless steel or nickel having about 200 lines per inch, or of a flat sheet of metal which has been etched or otherwise perforated to form openings therein. The openings 32 in the storage target 10 enable the flood electrons as well as the writing beam electrons to pass therethrough.

The halftone storage dielectric 30 may be a porous metal oxide, such as magnesium oxide or aluminum oxide, having a high resistivity and low density of less than 10 percent of its normal bulk density to provide such target with an extremely low capacitance. In one example, a target using a storage dielectric 30 of magnesium oxide having a density between 2 and percent of its normal bulk density and a thickness of up to 30 microns had a writing speed of about 300 million to 600 million centimeters per second, and a viewing time ofabout 1 second.

A second mesh electrode 34 is provided in spaced relationship to the first storage target on the side thereof remote from the second storage target 12. The mesh electrode 34 may have a larger mesh than target electrode 28 and functions as an ion repeller to prevent positive ions of residual gas from damaging the storage targets and, in some cases, may also function as a collector of secondary emission electrons. A collimating electrode 36 is provided as a ring-shaped coating of conducting material on the inner surface of the envelope 14 between the storage targets and the flood guns 24. The collimating electrode is connected to a positive D.C. voltage of +50 volts in order to collimate the low-velocity flood electrons so that they strike the bistable storage target 12 substantially perpendicular thereto. It should be noted that the collimating electrode 36 may in reality be a plurality of separate collimating electrodes of different potential.

As shown in FIG. 2, one embodiment of the bistable storage target 12 includes a storage dielectric 38 of phosphor material provided as an undivided layer over a light-transparent conductive film 40 of tin oxide or other suitable material coated on a light-transparent glass plate 42 which may be the faceplate of the cathode ray tube. The conductive film target electrode 40 serves as a collector electrode for the secondary electrons emitted from the phosphor storage dielectric 32. Thus, the undivided phosphor layer 38 is made sufficiently porous to enable secondary electrons to be transmitted therethrough so that such secondary electrons are emitted from the bombarded surface on the left side of the layer and are collected by the conductive film electrode 40 on the right-hand side of such layer. As a result, the second storage target 12 is capable of bistable storage of the charge image formed thereon. This bistable storage target is also described in US. Pat. No. 3,293,473 of R. H. Anderson, referred to above. It should be noted that the halftone storage dielectric 30 is much more porous than the phosphor storage dielectric 38 which has a density of about 50 percent of its normal bulk density so that the halftone storage target 10 has a lower capacitance than the bistable storage target.

The phosphor material of the bistable storage target 12 emits a light image corresponding to the charge image stored thereon which is viewed through the glass support plate 42 and the transparent conductive film 40. Any suitable high resistance phosphor material may be employed for storage dielectric 38, including zinc orthosilicate with a manganese activator (Zn,SiO,:Mn) known as P-l phosphor, or one of the zinc sulfide phosphors such as P31 phosphor. It should be noted that while the phosphor storage dielectric layer 38 is porous, it is still of high capacitance and relatively high density compared to the halftone storage dielectric 30. As a result, such phosphor dielectric is of slower writing speed but is capable of a long storage time, up to l hour or more.

The operation of the storage tube of the present invention is controlled by the four-position switches 44, 46 and 48 of FIG. I which are connected respectively to the ion repeller electrode 34, the first target electrode 28 and the second target electrode 40. In the first switch position labeled Bistable Storage," switch 44 applies a D.C. voltage of volts to the mesh electrode 34, switch 46 applies a D.C. voltage of +200 volts to the first target electrode 28, and switch 48 applies a D.C. voltage of+l 00 to +150 volts to the second target electrode 40. At these voltages, the electron beam emitted by the writing gun 16 passes through the mesh electrode 34 and the first storage target 10 and strikes the phosphor storage dielectric 38 of the second storage target to produce a charge image thereon. In addition, the flood electrons emitted by the flood gun 24 also pass through the mesh electrode 34 and the first storage target 10 due to their high positive voltage, and bombard the phosphor storage dielectric 38 to cause bistable storage of the charge image when the initial potential of such charge image exceeds the first crossover voltage of such dielectric. When using P-l phosphor as the storage dielectric 38, the first crossover voltage is about +50 volts so that for bistable storage the operating level potential of the target or collector electrode 40 is above such first crossover voltage but below the second crossover voltage of +200 volts. The flood electrons drive the potential of the charge image up to the voltage of the collector electrode 40 and drive the potential of the unwritten background areas down to the voltage of the flood gun cathode to cause bistable storage. The flood electrons also cause the phosphor storage dielectric to emit a light image corresponding to the stored charge image.

In order to form the charge image, the writing beam of electrons is deflected by deflection plates and 22 in the manner of a conventional cathode ray oscilloscope. Thus, an input signal whose waveform is to be stored is applied to an input terminal 50 and transmitted through a vertical amplifier 52 to the vertical deflection plates 22. A portion of this input signal is transmitted to a trigger generator circuit 54 which produces a corresponding trigger pulse that is applied to a horizontal ramp generator circuit 56 to cause such ramp generator to produce a ramp or sawtooth voltage which is applied to the horizontal deflection plates 20. The vertical amplifier 52 has a delay line to enable the ramp signal to be generated and then applied to the horizontal deflection plates at the same time the input signal is applied to the vertical deflection plates, as in the conventional trigger operation of an oscilloscope.

The storage tube of FIG. 1 is also capable of halftone storage operation by movement of the switches 44, 46 and 48 to the second position labeled "Halftone Storage" so that switch 44 applies a D.C. voltage of +l00 volts to mesh electrode 34, switch 46 applies a D.C. voltage of 20 volts to the first target electrode 28, and switch 48 applies a DC. voltage of +3 kilovolts to the second target electrode 40. At these voltages, the electron beam of the writing gun 16 forms a positivegoing charge image on the halftone storage dielectric 30 by secondary electron emission. The portion of the writing beam striking the bistable storage dielectric 38 does not produce a stored charge image due to the high voltage on the second target electrode 40. The flood electrons transmitted through the mesh electrode 34 pass through the apertures 32 in the first target electrode 28 and are modulated by the charge image on the halftone storage dielectric 30 so that a corresponding light image is produced on the phosphor layer 38. The light image is of extremely high brightness because the flood electrons are accelerated through an electrical field of 3 kilovolts. it should be noted that the surface of the halftone storage dielectric 30 is initially charged to a voltage near that of the 20 volts applied to the first target electrode 28 and is subsequently charged positive in the area of the charge image so that more flood electron current is transmitted through the mesh apertures 32 adjacent such positive image areas of the storage dielectric.

in the third position of switches 44, 46 and 48 labeled lmage Transfer," a DC. voltage of +1 50 volts is applied to the mesh electrode 34, a D.C. voltage of +100 to +l50 volts is applied to the second target electrode 40, and the first target electrode 28 is connected through an auxiliary switch 58 first to a voltage of -20 volts and subsequently to a voltage of +200 volts. In the first position of the auxiliary switch 28 labeled Halftone Storage," a charge image is written on the storage dielectric 30 of the first target 10, and when such switch is moved to the second position labeled Bistable Storage," this charge image is transferred to the phosphor storage dielectric 38 of the bistable storage target 12 by the flood electrons transmitted through the halftone target 10. As a result of this image transfer operation, the storage tube has an extremely fast writing speed, equal to that of the halftone storage target 10, and also has an extremely long storage time, equal to that of the bistable storage tube 12. it should be understood that the auxiliary switch 58 is not a manual switch but an electronic switch so that the charge image transfer is accomplished in a fraction of a second after such charge image is written on the halftone target. As a result, the charge image written on the halftone target need only be a few volts positive with respect to its unwritten background areas so that such halftone target has an even faster writing speed than would otherwise be possible without such image transfer.

In the fourth position of the switches 44, 46 and 48 labeled Erase," a D.C. voltage of+l 50 volts is applied to the ion repeller mesh electrode 34 while the target electrodes 28 and 40 are respectively connected by switches 46 and 48 to the outputs of an erase pulse generator 60. The erase pulse generator applies a positive voltage to the first target electrode 28 so that the storage dielectric 30 is charged uniformly positive by the flood electrons and then is returned to its quiescent voltage level of 20 volts. Similarly, the erase pulse generator applies a positive voltage pulse to the target electrode 40 of the bistable target so that the storage dielectric 38 is driven above its retention threshold voltage, approximately equal to the second crossover voltage, to cause the entire dielectric to charge uniformly positive and then such voltage is reduced below its retention threshold" voltage, approximately equal to the first crossover voltage, and then slowly returned to its quiescent voltage of+l00 to +150 volts. it should be noted that the quiescent operating level of the bistable storage target electrode 40 is between the retention threshold" voltage, below which storage is not possible, and the fade positive" voltage, above which storage is not possible.

Another embodiment of the bistable storage target used in the present storage tube is shown in FIGS. 3 and 3A as target 12'. Bistable storage target 12' includes a plurality of separate dots or areas 62 of phosphor material provided within the apertures of an apertured target electrode layer 64 of conducting material. The layer 64 may be of lighbtransparent conductive material, such as tin oxide, but may also be a light-opaque conductive material, such as aluminum. This embodiment has the advantage that the phosphor dots 62 may be made of greater thickness than the phosphor layer 38 of FIG. 2 and thereby provide a light image of greater brightness while still enabling bistable storage, as described in US. Pat. N0. 3,293,474 OF C. B. Gibson, issued Dec. 20, 1966.

A third embodimentof the bistable storage target is shown in FlGS. 4 and 4A as target 12". in this embodiment, the glass support plate 42 is etched on its inner surface to provide a plurality of conical glass projections 66. The projections 66 and the land areas between such projections are coated with a light-transparent conductive layer 68 of tin oxide and a bistable storage dielectric layer 70 of phosphor material is provided over such transparent conductive layer. The phosphor layer 70 is of a proper thickness such that the tips of the projections 66 extend through such layer to expose the portions of the transparent conductive coating 68 on such tips. As a result, the electrical field across the inner surface of the phosphor storage dielectric 70 is maintained substantially uniform so that the flood electrons always strike such dielectric substantially perpendicular thereto to prevent charge image spreading, as described in copending US. Pat. application, Ser. No. 619,904, now Pat. No. 3,53l,675, by R. A. Frankland, filed Feb. 28, I967.

The transparent conductive coating 68 of FIG. 4 functions as a collector electrode for the secondary electrons emitted from the phosphor layer 70, as does the mesh electrode 64 in FIG. 3 for the secondary electrons emitted by phosphor dots 62. Because of this use of the target electrode in the bistable target as the collector electrode, no additional collector electrode mesh is necessary between the two storage targets.

The storage tube of the present invention may also be operated in any other conventional mode previously used for the operation of a halftone storage tube, such as a variable persistence mode during which the storage time of the halftone charge image is varied by adjusting the time of application of the erase pulses to the target electrode 28. In addition, the storage tube may be operated in a write-through mode in which a nonstored charge image is formed on the bistable target adjacent to a stored charge image by either reducing the writing beam current density or duty cycle pulsing the writing beam to reduce the charge image voltage below the first crossover voltage. Also, electrical readout may be achieved by employing the writing gun 16 as a reading beam which is uniformly scanned across the surface of the bistable storage target by horizontal and vertical ramp signals of about 60 hertz and 15,750 hertz frequency, respectively, in the manner of a television raster.

It will be obvious to those having ordinary skill in the art that many changes may be made in the abovedescribed details of preferred embodiments of the invention without departing from the spirit of the invention. Therefore, the scope of the present invention can only be determined by the following claims.

We claim:

1. A charge transfer storage tube apparatus in which the improvement comprises:

a first storage target including a first target electrode of a mesh structure having a first storage dielectric of highly porous low density insulating material coated thereon so as to leave the mesh apertures open to enable the transmission of electrons therethrough;

a second storage target including a second target electrode and a second storage dielectric of higher density insulating material capable of bistable storage, said first dielectric being ofa more porous and lower density material than said second dielectric so that said first target has the lower capacitance and faster maximum stored writing speed than the second target while said second target has the longer storage time of the two targets;

writing means for producing a narrow beam of highvelocity electrons and for forming charge images on at least said first storage target with said beam; and

transfer and storage means for directing low-velocity electrons at said first target and transmitting at least some of them through the mesh apertures of said first target onto the second storage dielectric of said second target to enable a charge image formed on said first target to be transferred to said second target and stored thereon.

2. A storage tube apparatus in accordance with claim 1 in which the second storage target is a nonmesh target which has a second storage dielectric of phosphor material and has the second target electrode and said phosphor storage dielectric provided as layers on a light transparent support member of insulating material.

3. A storage tube apparatus in accordance with claim 1 in which the first storage dielectric is capable of halftone image storage and comprises a porous dielectric material of a high resistivity and a low density of less than 10 percent of its normal bulk density.

4. A storage tube apparatus in accordance with claim 3 in which the second storage dielectric is capable of bistable image storage by secondary electron emission when bombarded by said low-velocity electrons, and the transfer and storage means includes a collector means for collecting the secondary electrons emitted by said second dielectric.

5. A storage tube apparatus in accordance with claim 1 which also includes bias means for selectively applying different DC bias voltages to the first and second target electrodes to provide the storage tube with bistable storage, halftone storage, or nonstorage display operation as well as the aforementioned charge transfer operation.

6. A storage tube apparatus in accordance with claim 2 in which the second target electrode is a light transparent conductive film provided beneath the phosphor storage dielectric and the phosphor storage dielectric is an undivided phosphor layer which is sufficiently porous to enable secondary electrons emitted therefrom by the bombardment of low-velocity electrons to be transmitted through the phosphor layer and collected by said conductive film to enable bistable storage of charge images formed on said phosphor layer.

7. A storage tube apparatus in accordance with claim 3 in which the first storage dielectric is a porous metal oxide.

8. A storage tube apparatus in accordance with claim 7 in which the metal oxide is magnesium oxide.

9. A storage tube apparatus in accordance with claim 8 in which the magnesium oxide has a density of about 2 to 5 percent of its normal bulk density.

10. A storage tube apparatus in accordance with claim 1 which also includes a mesh electrode separate from the storage targets and supported on the side of the first storage target remote from said second target.

H. A storage tube apparatus in accordance with claim 2 in which the phosphor dielectric is provided as a plurality of separate, spaced phosphor areas.

12. A storage tube apparatus in accordance with claim I] in which the second target electrode is in the form of an apertured layer with said phosphor areas provided in the apertures of said layer.

13. A storage tube apparatus in accordance with claim 2 in which the transparent support member is a 14. A storage tube apparatus in accordance with claim 13 in which those portions of the conductive film covering the tips of said projections extend completely through the phosphor layer.

# i i l i 3 UNITED STATES PATENT OFFICE 5 CERTKFECATE Oi GORRECTKQN Patent No. 3,710 ,l73 Dated January 9 1973 Inventor Thomas B. Hutchins IV and William "1 'lemnleton 1 K It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

The title of the Letters Patent should --CHZ\RGE TRANSFER STORAGE TUBE HTVING LOW DENSITY HALFTOIIE TARGET AND HIGHER DENSITY BISTZBLE TARGET-- Signed and sealed this 29th day of May 1973.

[SEAL] Attest:

EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patent5 rnnll an 1nKn11nJZM 

2. A storage tube apparatus in accordance with claim 1 in which the second storage target is a nonmesh target which has a second storage dielectric of phosphor material and has the second target electrode and said phosphor storage dielectric provided as layers on a light transparent support member of insulating material.
 3. A storage tube apparatus in accordance with claim 1 in which the first storage dielectric is capable of halftone image storage and comprises a porous dielectric material of a high resistivity and a low density of less than 10 percent of its normal bulk density.
 4. A storage tube apparatus in accordance with claim 3 in which the second storage dielectric is capable of bistable image storage by secondary electron emission when bombarded by said low-velocity electrons, and the transfer and storage means includes a collector means for collecting the secondary electrons emitted by said second dielectric.
 5. A storage tube apparatus in accordance with claim 1 which also includes bias means for selectively applying different D.C. bias voltages to the first and second target electrodes to provide the storage tube with bistable storage, halftone storage, or nonstorage display operation as well as the aforementioned charge transfer operation.
 6. A storage tube apparatus in accordance with claim 2 in which the second target electrode is a light transparent conductive film provided beneath the phosphor storage dielectric and the phosphor storage dielectric is an undivided phosphor layer which is sufficiently porous to enable secondary electrons emitted therefrom by the bombardment of low-velocity electrons to be transmitted through the phosphor layer and collected by said conductive film to enable bistable storage of charge images formed on said phosphor layer.
 7. A storage tube apparatus in accordance with claim 3 in which the first storage dielectric is a porous metal oxide.
 8. A storage tube apparatus in accordance with claim 7 in which the metal oxide is magnesium oxide.
 9. A storage tube apparatus in accordance with claim 8 in which the magnesium oxide has a density of about 2 to 5 percent of its normal bulk density.
 10. A storage tube apparatus in accordance with claim 1 which also Includes a mesh electrode separate from the storage targets and supported on the side of the first storage target remote from said second target.
 11. A storage tube apparatus in accordance with claim 2 in which the phosphor dielectric is provided as a plurality of separate, spaced phosphor areas.
 12. A storage tube apparatus in accordance with claim 11 in which the second target electrode is in the form of an apertured layer with said phosphor areas provided in the apertures of said layer.
 13. A storage tube apparatus in accordance with claim 2 in which the transparent support member is a glass plate having a plurality of separate, spaced projections on one side thereof with land areas extending between said projections, the second target electrode is a transparent conductive film coated on said projections and said land areas, and the phosphor storage dielectric layer is coated over said conductive film.
 14. A storage tube apparatus in accordance with claim 13 in which those portions of the conductive film covering the tips of said projections extend completely through the phosphor layer. 